The present invention relates to an automatic transmission for a vehicle which uses a belt type continuously-variable speed transmission system (hereinafter referred to as "CVT").
The CVT includes an input pulley, an output pulley and an endless belt extended between the pulleys. The input pulley and the output pulley each includes a fixed sheave secured to a rotational shaft and a movable sheave mounted movably in an axial direction of the rotational shaft by means of a servo motor, and the movable sheave can be displaced axially to thereby change the ratio of rotational speed of the output pulley relative to that of the input pulley in the range of approximately 0.5 to 2.0.
When the CVT is used as the transmission for a vehicle, a first rotational shaft having the input pulley of CVT mounted thereon is connected to an output shaft of an engine through a coupling means, and between a second rotational shaft having the output pulley of CVT mounted thereon and a differential device are provided with a planetary gear mechanism and a forward-reverse changeover mechanism. A low-high speed changeover mechanism is provided in the planetary gear mechanism, if necessary. Generally speaking, the forward-reverse changeover mechanism and the low-high speed changeover mechanism are necessary to provide with a clutch for connecting specific elements of the planetary gear mechanism or a brake adapted to connect a specific element of the planetary gear mechanism in a stationary position. However, when a wet multiple-disc type clutch or brake is employed as the aforementioned clutch or brake, it is difficult to load the transmission on a small automobile because the diameter of a frictional engaging element of said wet multiple-disc type clutch or brake is large, resulting in a large dimension of the whole transmission for a vehicle.
What is needed is an automatic transmission for a vehicle wherein a forward-reverse changeover gear mechanism or a low-high speed changeover gear mechanism, which is provided between the rotational shaft having the output pulley of CVT mounted thereon and a transmission output shaft for a vehicle, is miniaturized and light in weight.